Polymer friction

Figure 10-17. "Friction polymer" at rubbing surfaces. (a) Wear scar on stationary ball of four-ball test, leading edge at bottom run in cyclohexane vapor. Fein and Kreuz [50]. (b) Wear scar on conically-ended pin, leading edge at bottom run in white oil. Dorinson, unpublished work. (c) Wear debris in polymer matrix as found in the lubricant. Dorinson, unpublished work.

When a chemically deposited film of "friction polymer" or "surface resin" derived from precursors intrinsically part of a petroleum oil acts as an additive in the high-pressure lubricant behavior of that oil, then the function of the oil must be viewed in a two-fold light. That part of the oil which generates the surface film is in effect an additive. The rest of the oil is the inert liquid carrier. 

Their mechanism can be presented as follows. During thermal treatment, FO contact the polymer melt and the oxides partially reduce to metals [72]. This leads to the generation of microchains M1-P-M2 in the lubricant bulk and formation of an MPE as a result of electrochemical interactions of unlike metal components. As a consequence, adhesive interactions between the filler and binder intensify. In our view, the high level of antiscoring properties and low friction coefEcient can be attributed to the catalyzing effect of the reduced metals on the formation of the friction polymers from the products of mechanical destruction of organic components of the lubricant. 

Surface-grafted, brushlike polymers can dramatically modify the lubricious properties of surfaces. The ability to bind a significant amount of solvent in a surface layer is thought to be one of the key mechanisms for low-friction, polymer-brush films. A brush composed of water-soluble, biocompatible polymers, such as poly(ethylene glycol), in an aqueous environment can provide an oil-free, environmentally friendly, food-compatible lubricious surface. 

UHMWPE tends to form transfer layers on metal coimterfaces in water through interlameUar shear of the polymer in the same way as other low-friction polymers, such as polytetrafluorethylene (PTFE). The low friction coefficients of these materials are attributed to the ease with which the polymer molecules shear against each other. Most other polymers show poor friction properties due to lumpy transfer of material to the metal surface. However, lumpy transfer, in which debris adheres to the metal surface, can also occur for PTFE or UHMWPE under certain conditions. For example, lumpy transfer of PTFE occurs at low sliding speeds and was shown to give a friction coefficient that was approximately twice that of the thin transfer fQm. ... 

Compared with rutile titanium dioxide, zinc sulphide offers white pigments for plastics that are non-abrasive, catalytically inactive, dry lubricants. They can reduce friction, polymer decomposition, and the wear of processing equipment. 

At high speeds (or low temperatures) these "low friction" polymers give a high friction and the transfer is much heavier, consisting of lumps or smeared films of polymer several lOOOX thick. The behavior in this regime may be determined by crystallite size, band structure or molecular weight or, in the case of polythene by spheruli te size. 

Effect on black plague corrosion on high temperature turbine alloys (718), on graphite corrosion in liq. CO2 (1111), on formation of frictional polymer (8 4-7), on reforming catalyst (577) ... 

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